Small Craft Advisory 398, NWS Anchorage, AK

AREA FORECAST DISCUSSION
SOUTHCENTRAL AND SOUTHWEST ALASKA
200 PM AKDT WED MAY 13 2009

.MODEL DISCUSSION...

THE BIGGEST ISSUE IN THE FORECAST PROCESS TODAY IS THE PROGRESSION OF
THE EASTWARD MOVEMENT OF THE WEATHER SYSTEM IN THE FAR WEST PACIFIC
AND ITS TIMING IN BREAKING DOWN THE RIDGE OVER THE BERING SEA RIDGE
PAST FRIDAY.  GENERALLY...PREFERRED TO STICK MORE WITH THE EXISTING
SLOWER MOVEMENT EASTWARD RATHER THAN LEAN TOWARDS THE NEW GFS WHICH
BREAKS DOWN THE RIDGE MORE QUICKLY.

.ANALYSIS AND UPPER LEVELS...

ALOFT...NORTHWESTERLY FLOW DOMINATES HIGH ABOVE MOST OF THE AREA OF
RESPONSIBILITY EARLY THIS AFTERNOON...EXCEPT FOR THE ALASKA PENINSULA
AND THE ALEUTIANS WHERE EASTERLY FLOW PERSISTS ON THE SOUTH SIDE OF
THE UPPER HIGH OVER THE NORTH CENTRAL BERING SEA.  A SHORTWAVE IS
MOVING THROUGH THE NORTHWESTERLY FLOW AND IS CURRENTLY APPROACHING
THE ALASKA RANGE FROM OVER THE UPPER KUSKOKWIM VALLEY...BUT ITS ONLY
EFFECTS ON THE WEATHER THUS FAR ARE TO PRODUCE SOME THIN WISPY
CIRRUS.

AT THE SURFACE...MOST OF SOUTHERN ALASKA IS ENJOYING ANOTHER
BEAUTIFUL AFTERNOON AND EVENING.  A THERMAL TROUGH EXTENDS ACROSS THE
COPPER RIVER BASIN INTO THE SOUTHERN SUSITNA VALLEY THIS AFTERNOON.
LOW STRATUS IS HANGING TIGHT JUST OFFSHORE EARLY THIS AFTERNOON IN
THE NORTHERN GULF...EXCEPT AT KODIAK WHERE FOG AND REDUCED
VISIBILITIES HAVE BEEN REPORTED RECENTLY.  NORTHEASTERLY FLOW AT THE
SURFACE PERSISTS ACROSS THE KUSKOKWIM RIVER VALLEY AND DELTA REGION
AROUND THE SOUTHEAST SIDE OF A SURFACE HIGH PRESSURE LOCATED OVER THE
EASTERN TIP OF THE SIBERIAN PENINSULA. THIS OFFSHORE FLOW IS
RESPONSIBLE FOR FAIR SKIES OVER THE SOUTHWEST COASTLINE...BRISTOL
BAY...AND THE SOUTHEAST BERING SEA.  WEAK HIGH PRESSURE PERSISTS
ACROSS THE SOUTHERN GULF OF ALASKA...WITH A WEAK WEATHER FRONT DRAPED
IN AN EAST-WEST FASHION ALONG OR JUST NORTH OF THE ALEUTIAN CHAIN
BETWEEN A 999 MB NORTH CENTRAL PACIFIC LOW AND A 988 MB LOW NORTHEAST
OF JAPAN IN THE NORTHWEST PACIFIC OCEAN.  THIS FRONT IS RESPONSIBLE
FOR SOME CLOUDS ACROSS THE WESTERN AND CENTRAL ALEUTIANS.
OTHERWISE...LOW STRATUS DOMINATES A LARGE PORTION OF THE CENTRAL
BERING SEA EXTENDING DOWN THROUGH MOST OF THE ALEUTIAN CHAIN
UNDERNEATH THE SUBSIDENCE INVERSION ALOFT.

.SHORT TERM FORECAST...

SOUTH AND EAST OF THE ALASKA RANGE...A SERIES OF SHORTWAVES WILL
PROGRESS SOUTHEASTWARD ACROSS THE AREA OF RESPONSIBILITY OVER THE
NEXT TWO DAYS.  THE SECOND OF THESE WHICH MOVES ACROSS LATE TONIGHT
AND TOMORROW WILL BEGIN TO INCREASE THE CHANCES FOR ISOLATED SHOWERS
ALONG THE CHUGACH AND WRANGELL-ST.ELIAS MOUNTAINS.  THE THIRD
SHORTWAVE BEGINS TO TRY TO PINCH OFF INTO A CLOSED LOW FRIDAY OVER
THE NORTHERN GULF AND WILL BRING INCREASED POPS TO THE PRINCE WILLIAM
SOUND AREAS PARTICULARLY IN THE HIGHER ELEVATIONS.

SOUTH AND WEST OF THE ALASKA RANGE...

THERE HAS NOT BEEN MUCH CHANGE TO THE GOING FORECAST...WITH MOSTLY
OFFSHORE FLOW KEEPING SKIES FAIR ALONG AND NEAR BRISTOL BAY OVER THE
NEXT THREE DAYS.  THE BIG QUESTION IS FIGURING OUT THE PROGRESS OF
THE NEXT SYSTEM AND ITS ASSOCIATED FRONT.  THE OFFICIAL FORECAST WILL
CALL FOR SLOWER PROGRESS EASTWARD OF THE FRONT THAN THE LATEST RUNS
OF THE GFS...WITH STRONG SOUTHERLY FLOW TO GALE FORCE PICKING UP IN
THE WESTERN ALEUTIANS TONIGHT AND THURSDAY...FOLLOWED BY MOVEMENT OF
THE FRONT THROUGH THE WESTERN ALEUTIANS FRIDAY NIGHT AND SATURDAY AND
THEN ENTERING THE CENTRAL BERING SEA AND CENTRAL ALEUTIANS SATURDAY
NIGHT.

.LONG TERM FORECAST...

LAST NIGHTS GEFS ENSEMBLE MODELS RELATIVE MEASURE OF PREDICTABILITY
TOOL SHOWS LITTLE PREDICTABILITY FROM SUNDAY ONWARD...AND THIS IS
LIKELY DUE TO THE POOR HANDLING OF THE EVOLUTION OF THE RIDGE
BREAKDOWN IN THE BERING SEA BY THE GFS MODEL IN PARTICULAR.
HOWEVER...THERE IS HIGH PREDICTABILITY THAT A RIDGE WILL BUILD ACROSS
THE WESTERN BERING SEA TOWARDS THE MIDDLE OF NEXT WEEK...SO THE FRONT
AND UPPER TROUGH ARE LIKELY TO BE IN THE EASTERN BERING SEA OR ALONG
THE SOUTHWEST COASTLINE SOMETIME MONDAY THROUGH WEDNESDAY...BRINGING
AN INCREASED CHANCE OF PRECIPITATION TO THE STATE DEPENDING ON THE
STRENGTH AND EVOLUTION OF THE LOW/TROUGH.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...FLOOD WARNING...155.
MARINE...GALE...176.
FIRE WEATHER...NONE.

CC MAY 09